In recent years, printed circuit components have become widely used in a variety of electronic devices. Of particular interest are multi-layer printed circuit board laminates which have been developed to meet the demand for miniaturization of electronic components and the need for printed circuit boards having a high density of electrical interconnections and circuitry. In the manufacture of multi-layer printed circuit boards, conductive foils, which are usually copper foils, are secured to opposite sides of a core which is conventionally a reinforced or non-reinforced dielectric. (Throughout this specification, the use of the term "core" is meant to include any one of a variety of core materials, all of which may be reinforced or non-reinforced and may include an epoxy, polyester, polyimide, a polytetrafloroethylene, and in some applications, a core material which includes previously formed printed circuits).
The process includes one or more etching steps in which the undesired or unwanted copper is removed by etching away portions of the conductive foil from the laminate surface to leave a distinct pattern of conductive lines and formed elements on the surface of the etched laminate. The etched laminate and other laminate materials may then be packaged together to form a multi-layer circuit board package. Additional processing, such as hole drilling and component attaching, will eventually complete the printed circuit board product.
The trend in recent years has been to reduce the size of electronic components and provide printed circuit boards having multi-chip modules, etc. This results in a need to increase the number of components, such as surface-mount components provided on the printed circuit board. This in turn results in a so-called "densely populated" or simply "dense" printed circuit board. A key to providing a densely populated printed circuit board is to produce close and fine circuit patterns on the outer surfaces (i.e., the exposed surfaces) of the resulting multi-layer printed circuit board. The width and spacing of conductive paths on a printed circuit board are generally dictated by the thickness of the copper foil used thereon. For example, if the copper foil has a thickness of 35 .mu.m (which is a conventional 1-ounce foil used in the manufacture of many printed circuits), exposing the printed circuit board to an etching process for a period of time to remove such a foil thickness will also reduce the width of the side areas of the printed circuit path in approximately the same amount. In other words, because of the original thickness of the copper foil, a printed circuit board must be designed to take into account that an etching process will also eat away the sides of a circuit path (i.e., undercut a masking material). In other words, the thickness of the spacings between adjacent circuit lines is basically limited by the thickness of the copper foil used on the outer surface of the multi-layer printed circuit board.
Thus, to produce "densely populated" printed circuit boards, it is necessary to reduce the thickness of the copper, at least on the outermost surface of the multi-layer printed circuit package. (The thickness of the copper foil sheet is generally limited by the ability of a foil manufacturer to handle and transport such sheets. In this respect, as the thickness of the foil decreases below 35 .mu.m, the ability to physically handle such foil becomes more difficult).
Many printed circuit boards also include conductive layers containing patterned components that perform as specific, discrete components. One such discrete component is a resistive element. It is conventionally known to form a resistive element using a resistor foil. A resistor foil is basically a copper foil having a thin layer of a resistive material, typically a metal or metal alloy, deposited onto one surface thereof. The resistor foil is attached to a dielectric substrate with the resistor material being adhered to the dielectric substrate. Portions of the copper foil and resistive material are etched away, using conventionally known etching and masking techniques, to produce a trace line comprised of copper and the resistive material therebelow. A section of the copper layer is removed leaving only a resistive material trace line remaining on the surface of the dielectric to connect the two separated ends of the copper portion of the trace line. Because the resistive material typically has a conductivity less than copper, it essentially acts as a resistor between the separated ends of the copper portion of the trace line. As will be appreciated, the foregoing subtractive procedure requires several masking and etching steps to remove unwanted copper and resistive material to form the actual resistive element. Such steps are both time-consuming and expensive. Further, the resistive materials used in forming the resistor foil are somewhat limited to those materials that can be etched using known etching chemicals. In this respect, the resistive material must be material that is compatible with chemicals used to etch copper.
The present invention provides an outer surface component for forming resistive elements in a multi-layer printed circuit board and a method of forming embedded resistive elements in a multi-layer printed circuit board that utilizes a process that is not limited by known resistive materials.